The present invention relates to a low energy fluid purification system. In particular, apparatus and methods are provided that allows natural temperature gradients in the ocean to be used as the heat source and sink for evaporating non-potable, e.g. saline, water under vacuum pressures and condensation of potable water. Various embodiments of the invention facilitate production of potable water with limited man-made energy input, thus minimizing the cost of the water production. Because of the vastness of the ocean, the thermal gradients represent a quasi-infinite heat source and heat sink.
Previous methods to distill ocean water have not been economically viable for multiple reasons. One major reason is for the lack of economic viability arising from a need for enormous quantities of energy required to boil and condense water. Various embodiments can utilize a way to use naturally occurring thermal gradients in the ocean to transfer the energy to and from the water thus saving the cost of having to input the energy from manmade sources. Another source of failure is unnecessary complexity. Embodiments of the invention can include designs that limit a number of valves and moving parts and uses natural pressure gradients to minimize the work required to pump water. Designing a system to maximize the work the environment does and minimize work and energy input from manmade sources decreases complexity, decreases cost, decreases maintenance and improves the chance of success.
Some background to some embodiments of the invention can include design approaches noting that a temperature at which water boils is pressure dependent (lower pressure=lower boiling temp). A pressure at which water condenses is temperature dependent (lower condensation temp=lower pressure). Temperature of water near the surface of the ocean is warmer than water deep in the ocean. One gallon of water can be boiled by inputting approximately 9,000,000 joules of energy. One gallon of water vapor can be condensed to potable water by removing approximately 9,000,000 joules of energy. Approximately 9,000,000 joules of energy is required to raise the temperature of 200 gallons of water by 2.7 degrees Celsius.
Generally, one embodiment of the invention can include a low energy fluid purification system including a first vacuum-rated chamber extending above a body of water. The evaporation chamber holds a column of water at a sufficient height so as to create a low pressure area above the column of water. Due to the low pressure, the ambient temperature of the held water is sufficient to vaporize water at an upper surface of the column of water. A gas transfer structure is coupled to the first chamber so as to convey the vaporized water away from the first chamber. A second vacuum-rated chamber is coupled to the gas transfer structure and receives the vaporized water. A condensation system is positioned within the condensation chamber configured to receive water from a depth of the body of water that is sufficiently cool to be used as a cooling fluid for the condensation system, and a condensation collection system positioned to capture condensed water produced from the condensation system. Other elements can include systems for making use of gravity or siphoning effects for transferring water within the system and a movement system.
More particularly according to one illustrative simplified embodiment of the present disclosure, some basic components of the invention can include a first-vacuum rated chamber (e.g. evaporation chamber), connected to a second vacuum-rated chamber (e.g. condensation chamber). The dual chamber system is initially evacuated of gas to create vacuum pressure whereupon the water in the evaporation chamber will begin to boil. Vapor from the evaporation chamber will move to the condensing chamber and condense. The exemplary evaporation chamber must be maintained at a higher temperature than the condensation chamber. A temperature difference will cause a pressure difference and vapor flow will not require any additional means. As water in the evaporation chamber transitions to vapor, the salinity of the water in the evaporation chamber increases and a means to exchange lower salinity ocean water for higher salinity water in the evaporation chamber is required. A means to transfer heat to the evaporation chamber is required. A system to conduct heat away from the condensing chamber is required. A system to initially evacuate gas from the two chamber system is required. A system to remove potable water from the evacuated condensing chamber is required.
Embodiments of the invention can be optimal for locations where ocean thermal gradients are not large as a function of depth. The depth vs temperature profile of such a location is depicted in FIG. 1. Note that in the example shown in FIG. 1, minimal temperature gradients are observed for the first 300 meters and a temperature drop from about 22 C to 8 C is observed between 300 and 750 meters. Other embodiments of the invention can be optimized for locations where ocean thermal gradients are large as a function of depth (i.e. a large temperature differential for little change in depth).
Various types of embodiments of the invention can be designed to be optimal for locations where ocean thermal gradients are large as a function of depth. In the previous discussion of the embodiment of the invention optimized for low thermal gradients as a function of depth (regions where large depths were required to access low temperature ocean water), the inventors relied on similar thermodynamics but a different apparatus. In the low thermal gradient embodiment, pumps were proposed to move large quantities of ocean water from large depths to achieve the required cooling. The high thermal gradient embodiment uses a passive system and relies on natural convection and ocean currents to generate the thermodynamic driving force for heat exchange.
The cross over point for efficiency of large thermal gradient system vs. the small thermal gradient can be calculated. That is, given a particular thermal gradient and corresponding depth to achieve that temperature difference one of the two systems would have a higher efficiency, due to their various designs, as illustrated in the following example: a designer can assume a 10 degree Fahrenheit temperature difference between hot and cold water is required for commercial operation of the exemplary low energy fluid purification system. Further assume that this temperature differential is achievable by using water from the surface of the ocean and water from 30 feet down in the ocean. The large thermal gradient system would require pumping each gallon of potable water from a depth of 30 feet against vacuum pressure. Pumping water against vacuum pressure adds an effective 33 feet additional height resulting in an energy input to pump 63 foot gallons of water per gallon of distilled water. The small thermal gradient system would require an additional assumption. Assume that the water used as the heat sink and heat source changed temperature by 2.7 degrees during the heat transfer processes. This would require 200 gallons of surface water and 200 gallons of deep cold water to be pumped for each gallon of distilled water produced. Further assume the siphon and vacuum assistance limited the work the pumps had to do to only 1 foot of effective pumping distance. This would result in an energy input to pump 401 foot gallons of water per gallon of distilled water. With these assumptions, the large thermal gradient system would be more efficient.
Using one change in the assumptions, the small thermal gradient system becomes more efficient. Assume the 10 F temperature gradient requires 500 feet depth water. In this scenario, the large thermal gradient system requires energy to pump 533 foot gallons of water per gallon of distilled water. The small thermal gradient system requires the same energy to pump 401 foot gallons of water per gallon of distilled water because the siphon and vacuum effects are nearly independent of depth. The current exemplary discussion has made various assumptions which may be inaccurate in some cases but a crossover point exists and will be determined by local thermal gradients, temperature change achieved in heat source and sink, and the materials used to construct the apparatus.
Initially this technology can be explored for localities where potable water is most expensive such as islands where water must be brought in by barges. Secondly the technology would be most useful for coastal communities. One bonus of the technology is that because efficiency increases with larger temperature gradients if the depth remains constant, potential global warming will improve the process by raising the temperature of the surface water of the ocean. This technology will compete with current desalination methods including reverse osmosis, and other vacuum and distillation methods.
The use of “water” to describe the fluid to be purified is not meant to restrict application to a particular fluid. Embodiments of the invention can be used to purify a liquid wherein any contaminants require a higher vaporization temperature than the liquid to be purified at a given pressure.
Additional features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of the illustrative embodiment exemplifying the best mode of carrying out the invention as presently perceived.